Gas turbine engines are known to include a compressor for compressing air, a combustor for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor, and a turbine for expanding the hot gas to extract shaft power. The combustion process in many older gas turbine engines is dominated by diffusion type nozzles producing flames burning at or near stoichiometric conditions with flame temperatures exceeding 3,000° F. Such combustion will produce a high level of oxides of nitrogen (NOx). Current emissions regulations have greatly reduced the allowable levels of NOx emissions. Lean premix type combustion has been developed to reduce the peak flame temperatures and to correspondingly reduce the production of NOx in gas turbine engines. In a premixed combustion process, fuel and air are premixed in a premixing section of the combustor. The fuel-air mixture is then introduced into a combustion chamber where it is burned. U.S. Pat. No. 6,082,111 describes a gas turbine engine utilizing a can annular premix combustor design. Multiple premix-type nozzle assemblies are positioned in a ring to provide a premixed fuel/air mixture to a combustion chamber. A diffusion type pilot fuel nozzle assembly is located at the center of the ring to provide a flow of pilot fuel to the combustion chamber.
The design of a gas turbine combustor is complicated by the necessity for the gas turbine engine to operate reliably with a low level of emissions at a variety of power levels and during a variety of operating sequences. High power operation at high firing temperatures tends to increase the generation of oxides of nitrogen. Low power operation at lower combustion temperatures tends to increase the generation of carbon monoxide and unburned hydrocarbons due to incomplete combustion of the fuel. Under all operating conditions and sequences, it is important to ensure the stability of the flame to avoid unexpected flameout, damaging levels of acoustic vibration, and damaging flashback of the flame from the combustion chamber into the fuel premix section of the combustor. A relatively rich fuel/air mixture, such as provided by a diffusion type nozzle, will improve the stability of the combustion process but will have an adverse affect on the level of emissions. A diffusion type nozzle may be especially useful to maintain flame stability during operations in which a rapid decrease in fueling is effectuated. In view of these emissions concerns and operations considerations, a careful balance must be achieved among these various constraints in order to provide a reliable machine capable of satisfying very strict modern emissions regulations.
Whereas older pilot nozzle assemblies often were diffusion type nozzles, newer styles include premix capability. Some such newer nozzle assemblies may additionally comprise a diffusion capability, but with that dual capability comes a higher initial cost and subsequent durability issues, such as due to the presence of baffles or other features that address thermal expansion issues among the components. Such greater complexity, in addition to increasing initial cost, may lead to less overall reliability. Other such newer nozzle assemblies may have a lower initial cost but may not provide diffusion capability, nor an interchangeable (with regard to fuel type and function) diffusion capability. The present invention advances the art by addressing these concerns.